Physiological solutions stored in plastic bags need to be pressurized when infusion of the solutions is inhibited by either high solution viscosity (e.g., pack red blood cells), high pressure (e.g., infusion into the arterial tree), or when rapid infusion is required (e.g. crystalloid cardioplegia, hypovolumic shock). In addition, flush solutions used during invasive pressure measurements need to be pressurized to a constant pressure in order to maintain a constant infusion to keep the intravascular catheter patent. There are presently several types of devices that are commercially available to pressurize bags. The most popular is a two compartment soft bag with an inflatable bladder on one side and a pocket on the other side. The bag to be pressurized is inserted in the pocket. The bladder is inflated with air via a hand pump, tubing and valves. As the bladder inflates, it imparts external compression on the encased bag in the pocket. A pressure gauge indicates the pressure within the bladder. This type of device is illustrated in U.S. Pat. No. 4,090,514.
A second type of device available is a non-disposable rigid housing into which the solution bag is inserted. The bag is compressed by a spring-loaded plate which also supports the bag within the housing. The spring-loading is engaged by a mechanical crank mounted on the box. This type of device is illustrated in U.S. Pat. No. 3,902,635. A third type of device is similar to the first two types except that it incorporates a pressure regulator that maintains the inflatable bladder at a constant pressure. The unit requires an external gas source (e.g. "wall-supplied" oxygen) and is very expensive.
These commercially available devices have the following problems:
1. Cost - All, except one, are non-disposable, and relatively expensive. A variety of non-repairable malfunctions make the life expectancy of these devices short.
2. Sterility - They are used with blood, blood products and intravenous solutions in high risk areas of the hospital where cross contamination between patients is a prime consideration. Washing and/or sterilization of the non-disposable units is not easy and can lead to failure of these units.
3. Accuracy - Inaccurate infusion pressure occurs as the physiological solution is infused. The soft system, intermittently has to be manually compressed to maintain the same compression pressure against the bag with the physiological solution. Even when the compression pressure is maintained constant, the infusion pressure decreases as the volume in the bag decreases. Therefore, proper bladder pressure, as indicated on the pressure gauges, does not assure proper infusion pressure. The prior art systems require appropriate initial over pressurization, higher than 400 mmHg, to maintain the infusion pressure above 250 mmHg, as the volume of the bag changes from full to empty. When the bag's volume is emptied to 100 ml, the pressure may be as low as 200 mmHg. Additional inaccuracies may occur with (a) inappropriate placement of the bag within the compression devices causing herniation and solution trapping, a decrease in infusion pressure and decrease in infusion rate; (b) malfunctioning gauges - the gauges are usually not calibrated and are usually discarded only if "stuck". In fact the manufacturer of one of the most widely used pressure cuffs warns against the use of the cuff for use in monitoring arterial or venous pressure.
4. Safety - A decrease in infusion rate can be detrimental to the patient in two ways. First, the patient does not get the required solutions at the rate set initially unless an adjustment is made in the flow controlling resistor located in the I.V. line. Second, when heparinized solutions are used for continuous flush of intravascular catheters, a decrease in flush solution can result in formation of thrombus at the catheter tip which may lead to vessel damage and inaccurate pressure reading. The latter can result in an inappropriate therapy. The rigid device also poses a possible safety hazard to a patient lying under it. Should the device fall, patient injury can result.
In addition to the devices that are currently commercially available, the prior art includes a number of patents which describe constant pressure infusion pumps which include both ambulatory devices, and fixed location devices intended to be attached to hospital air supplies.
U.S. Pat. No. 3,468,308 discloses a pressure infusion device for ambulatory patients with a pressure control means. FIGS. 10-12 of this reference disclose an infusion device having a portable CO.sub.2 cartridge, a separate pressure regulator, a pressure chamber, and a bladder for discharging a solution into an ambulatory patient This device, however, is not intended to be a low cost disposable device which may be disposed of to avoid cross-contamination of patients. In addition, it is not capable of receiving standard disposable bags of physiological solution. The saline bladder must be refilled after each discharge.
U.S. Pat. No. 2,842,123 and 3,044,663, disclose pressurized transfusion devices that are particularly adapted to be connected to a compressed air line available in most hospitals. These devices are not disposable low cost devices, nor are they capable of accepting or discharging the contents of disposable bags of physiological solution.
U.S. Pat. No. 4,048,994 describes a self inflating liquid container for physiological solutions that may be disposed of. The devices are adapted to be pressurized by a sealed cartridge of CO.sub.2, but it is not a constant pressure device, nor may it be reused with disposable bags of sterile physiological solutions.
U.S. Pat. No. 4,507,116 discloses a device particularly adapted for receiving and discharging the contents of a disposable bag of sterile physiological solutions. The device includes an accumulator 20 which is intended to be dilated with excess pressure pumped by bulb 10 to maintain the discharge pressure of the device at a somewhat constant level. The device uses a relatively linear portion along the curve of elastic deformation of the bladder to provide its constant discharge pressure. While conceptually feasible, the device would require an accumulator having the capacity to store, at the desired pressure-volume ratios, the amount of air necessary to discharge 500 to 1000 ml of solution, before a constant pressure could be achieved for the entire bag of solution. This would result in a rather bulky and expensive device with the volume of the accumulator approaching that of the infusion device.
U.S. Pat. Nos. 3,506,005 and 4,419,096 disclose constant pressure infusion devices, wherein the constant pressure is supplied by elastomatic bladders that are expanded along a linear portion of their curves of elastic deformation to achieve their constant pressure discharge.
U.S. Pat. No. 4,379,543 discloses an infusion device particularly adapted for use with disposable bags of sterile physiological solution, wherein the pressure generating means for discharging this solution is supplied by a pair of chemical agents which generate a gas when mixed. However, once activated, the device generates a fixed amount of compressed gas, and the discharge of the physiological solution for the bag results in a declining pressure for the discharge solution. The device does not utilize any pressure regulating means. Similar gas generating devices intended for use in the medical field are also disclosed in U.S. Pat. Nos. 4,203,441 and 4,300,542.
Since the currently available prior art devices are expensive, and not disposable or if disposable, are inaccurate and possibly dangerous to the patient, it is felt that a device which would overcome these problems would be both new and useful.